The invention is related to an apparatus and method for applying varying pressure waveforms to a limb of a human patient in order to help prevent deep vein thrombosis (DVT), pulmonary embolism (PE) and death.
Limb compression systems of the prior art apply and release pressure on a patient""s extremity to augment venous blood flow and help prevent deep vein thrombosis (DVT), pulmonary embolism (PE) and death. Limb compression systems of the prior art typically include: a source of pressurized gas; one or more pneumatic sleeves for attaching to one or both of the lower limbs of a patient; and an instrument connected to the source of pressurized gas and connected to the sleeves by means of pneumatic tubing, for controlling the inflation and deflation of the sleeves and their periods of inflation and deflation. In U.S. Pat. No. 3,892,229 Taylor et al. describe an early example of one general type of limb compression system of the prior art known as an intermittent limb compression system; such systems apply pressure intermittently to each limb by inflating and deflating a single-bladder sleeve attached to the limb. In U.S. Pat. No. 4,013,069 Hasty describes an example of a second general type of limb compression system of the prior art, known as a sequential limb compression system; such systems apply pressure sequentially along the length of the limb by means of a multiple-bladder sleeve or multiple sleeves attached to the same limb which are inflated and deflated at different times. Certain intermittent and sequential limb compression systems of the prior art are designed to inflate and deflate sleeves thereby producing pressure waveforms to be applied to both limbs either simultaneously or alternately, while others are designed to produce pressure waveforms for application to one limb only.
One major concern with all pneumatic limb compression systems of the prior art is that the therapy actually delivered by these systems may vary substantially from the expected compression therapy. For example, a recent clinical study designed by one of the inventors of the present invention, and involving the most commonly used sequential pneumatic limb compression systems of the prior art, showed that the pneumatic limb compression therapy actually delivered to 49 patients following elective total hip replacement surgery varied widely from therapy expected by the operating surgeons in respect of key parameters of the therapy shown in the clinical literature to affect patient outcomes related to the incidence of deep venous thrombosis, pulmonary embolism and death. The study methodology involved continuous monitoring of the varying pressure of the compressed air in the pneumatic sleeves of these systems, permitting the values of key parameters of pneumatic compression therapy actually delivered to patients to be directly monitored throughout the prescribed period of therapy and compared to the expectations of operating surgeons. The results of this clinical study indicated that the expected therapy was not delivered to any of the 49 patients monitored: therapy was only delivered an average of 77.8 percent of the time during the expected periods of therapy; the longest interruptions of therapy in individual subjects averaged 9.3 hr; and during 99.9 percent of the expected therapy times for all 49 patients monitored in the study, values of key outcomes-related parameters of the therapy actually delivered to the patients varied by more than 10 percent from desired values. These parameters included rates of pressure rise and maximum pressures actually delivered through the sleeves. The unanticipated range of variations that was found in this clinical study between expected and delivered pneumatic compression therapy, within individual patients and across all patients, may be an important source of variations in patient outcomes in respect of the incidence of deep vein thrombosis, pulmonary embolism and death, and may be an important confounding variable in comparatively evaluating reports of those patient outcomes. The present invention addresses many of the limitations of prior-art systems that have led to such unanticipated and wide variations between the expected therapy and the therapy actually delivered to patients.
Due to errors and limitations associated with estimation of the pressure applied by a sleeve to a limb, prior-art systems have not had the capability of accurately producing a desired pressure waveform in combination with sleeves having differing designs and varying pneumatic volumes, or when sleeve application techniques vary and the resulting sleeve snugness varies, or when sleeves are applied to limbs of differing sizes, shapes and tissue characteristics. As a result, substantial variations often arise between the desired and actual pressure waveforms delivered by limb compression systems of the prior art.
Many limb compression systems of the prior art are not capable of producing a desired pressure waveform in a pneumatic sleeve attached to a limb under varying operational and clinical circumstances such as movement of the limb, movement of the sleeve relative to the limb and varying snugness of sleeve application, in part because they do not generate a signal indicative of the actual pressure in the sleeve suitable for permitting a feedback control system to produce the desired pressure waveform. Some limb compression systems known in the prior art attempt to estimate sleeve pressure in an inexpensive and convenient manner, based on a variety of apparatus and methods. These systems do not measure pressure directly in the pneumatic sleeve applied to the limb but instead estimate sleeve pressure indirectly and remotely from the sleeve. For example, in U.S. Pat. No. 5,031,604 Dye describes a system in which sleeve pressure is estimated by measuring pneumatic pressure near the instrument end of the tubing connecting the instrument to the sleeve. As another example, Arkans in U.S. Pat. No. 4,375,217 describes a system in which the static pressure in the sleeve is estimated at a location on the tubing between the instrument and the sleeve. All such apparatus and methods which estimate sleeve pressure by measuring a pneumatic pressure remotely from the sleeve suffer from a significant disadvantage, which makes them unsuitable for incorporation into an instrument for producing a desired pressure waveform in the sleeve: the accuracy of the estimates of pressure made by such systems is significantly affected by variations in the length and flow resistance of the tubing attached to the sleeve, and by variations in sleeve design, sleeve inflation volume and sleeve application technique. For example, the inventors of the present invention have determined that variables related to the design and size of the sleeve, as well as the snugness of application of the sleeve, can result in discrepancies at any instant of well over 50 percent between the remotely estimated sleeve pressure and the actual pressure in the sleeve. As a separate consideration regarding the flow resistance of the tubing employed in prior-art systems which measure pressure in this manner, it has been necessary to locate such systems close to the patient to minimize flow resistance in the tubing, resulting in unnecessary noise and clutter around the patient.
Other systems known in the prior art interrupt the flow of gas in the tubing in an effort to estimate sleeve pressure by measuring pneumatic pressure at the instrument end of the tubing under zero-flow conditions. One such system is the Jobst Athrombic Pump System 2500 (Jobst Institute Inc., Charlotte N.C.). However, estimates of sleeve pressure made in this manner cannot practically be incorporated into limb compression systems for producing pressure waveforms having large amplitudes and short cycle periods. Also, more generally, such systems suffer from the disadvantage that pressure estimates are available discontinuously and are not suitable for real-time control of the pressure in the sleeve to produce a desired pressure waveform.
Some limb compression systems of the prior art attempt to record and display the total cumulative time during which pneumatic compression therapy was delivered to a patient""s limb, but do not differentiate between times when values of parameters of the delivered therapy were near the desired values for the therapy and when they were not. For example, commercially available systems such as system the Plexipulse intermittent pneumatic compression device (NuTech, San Antonio Tex.) and AirCast intermittent pneumatic compression device (Aircast Inc., Summit, N.J.) record the cumulative time that compressed air was delivered to each compression sleeve. These are typical of prior-art systems which include simple timers that record merely the cumulative time that the systems were in operation.
In U.S. Pat. No. 5,443,440 Tumey et al. describe a pneumatic limb compression system capable of recording compliance data by creating and storing the time, date and duration of each use of the system for subsequent transmission to a physician""s computer. The compliance information recorded by this system contains only information relating to times when the system was operating and the cumulative duration of operation. Tumey et al. cannot and does not determine occurrences when pressure-related values of parameters of the delivered therapy matched the desired values of the parameters and occurrences when they did not.
A major limitation of Tumey et al. and other limb compression systems of the prior art is that values of key parameters of pneumatic compression therapy that are known to affect patient outcomes are not monitored and recorded. This is a serious limitation because evidence in the clinical literature shows that variations in applied pressure waveforms produce substantial variations in venous blood flow, and that delays and interruptions in the delivery of pneumatic compression therapy affect the incidence of DVT. One key parameter identified by the inventors of the present invention is the interval between successive occurrences of delivered pressure waveforms having desired values of certain waveform parameters known to affect patient outcomes, such as rate of pressure rise and maximum pressure. Because this key parameter is not monitored as therapy is delivered by prior-art systems, variations between delivered and expected therapy cannot be detected as they occur, and clinical staff and patients cannot be alerted to take corrective measures for improving therapy and patient outcomes.
Because prior-art systems do not monitor the interval between successive occurrences of delivered pressure waveforms having desired values of certain waveform parameters known to affect patient outcomes, and because such prior-art systems do not therefore have alarms to alert clinicians and patients that a maximum time interval has elapsed during which the expected therapy was not delivered to the patient, then the operator and the patient cannot adapt such systems during therapy, including for example sleeve re-application and changing certain parameters of therapy, to help assure that the prescribed and expected therapy is actually delivered to the patient throughout as much as possible of the prescribed duration of therapy.
In addition to the monitoring limitations of prior-art systems described above, prior art systems do not measure and record parameters related to the application of a desired pressure waveform, such as any differences between the actual shape of the pressure waveform produced in the pneumatic sleeve and the shape of a desired reference pressure waveform, the times during which a waveform matching a desired waveform in respect of key parameters was periodically applied, the interval between applications of waveforms matching a desired waveform and the number of cycles of the waveform which were applied.
Additionally, limb compression systems do not subsequently produce the recorded values of key outcomes-related parameters for use by physicians and others in determining the extent to which the prescribed and desired pressure waveforms were actually applied to the patient for use by third-party payors in reimbursing for therapy actually provided, and for use in improving patient outcomes by reducing variations in parameters of therapy known to produce variations in patient outcomes.
The present invention provides apparatus and a method for applying pressure to a patient""s limb through a pneumatic sleeve in order to augment venous blood flow in the limb and for monitoring the applied pressure, to help prevent deep vein thrombosis, pulmonary embolism and death. More specifically, the present invention includes means for supplying a gas at a varying supply pressure, an inflatable sleeve adapted for positioning onto a limb to apply a varying pressure to the limb beneath the sleeve when inflated with the gas, pressure transducing means for measuring the pressure of gas in the inflatable sleeve, waveform parameter measurement means for measuring the value of a predetermined pressure waveform parameter, and interval determination means for producing an indication of the interval between two occurrences when the measured value of the predetermined pressure waveform parameter is near a predetermined parameter level.
In the present invention, the pressure waveform parameter can be a predetermined variation in the measured level of pressure of gas in the sleeve that augments the flow of venous blood into the limb proximal to the sleeve from the limb beneath the sleeve. Also, the sleeve of the present invention can include two ports and separate tubing connecting it to the gas supply means and the pressure transducing means so that the pressure transducing means only communicates pneumatically with the gas supply means through the sleeve.
The present invention includes means to allow an operator to select the predetermined pressure waveform parameter and the predetermined parameter level from a plurality of predefined parameters and parameter levels. Also, alarm means are included for producing an indication perceptible to the operator and the patient when the determined interval exceeds a predetermined maximum interval.
The interval determination means of the present invention can include means for measuring a number of intervals during therapy, each corresponding to the time between an occurrence when the measured value of the parameter is near the predetermined parameter level and the next occurrence when the measured value of the parameter is near the predetermined parameter level. The interval determination means can further include a clock for determining the clock times when occurrences are measured.